Secret signaling



Feb. 23 1926.

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H. FLETCHER SECRET SIGNALING Filed July 31 5 SheebS-Sheefn 2 /m/en/o/ /7af Vey Hefe/7er by 427)( Feb. 23 1926.

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HARVEY FLETCHER, 0E NEW YORK, N. Y., ASSIGNOB, T0 WESTERN ELECTRIC?y GOM- .IPANY, INCBPOBATED, QF NEW YORK, N. Y., A CORPORATION 0F NEW YORK.

SECRET SIGNALING.

Application led July 31, 1923, Serial No. 654,803.

To all whom it may concern.'

Be it known that I, HARVEY FLETCHER, a citizen of the United States of America, residing at New York city, in the county of New York and State of New York, have invented certain new and useful Improve-v ments in Secret Signaling, of which the following is a full, clear, concise, andexact description.

The present invention relates to wave transmission and reception; and more particularly has to do with the securing of secrecy in the transmission of waves for any purpose such as signaling.

It has been proposed to secure secrecy of wave transmission by sub-dividing the waves, such as speech waves, for exam le, into. sub-bands of relatively narrow requency ranges and displacing the individual bands or inverting the frequency order within the individual sub-bands or otherwise operating on the sub-bands to render very difficult the restoration of the transmitted waves to their original form. Such systems rely for secrecy on the fact that it is impossible to restore the waves to their original form by means of the ordinary types of receivers even if the secrecy scheme be` known. Special apparatus, such as wave filters, designed to have the proper frequency transmission characteristics together with wave combining circuits, are generally required. vlf the scheme of transforming the Waves to their unrecognizable condition were known andif this same scheme were invariably used, it is conceivable that an unauthorized person of sufficient patience and resourcefulness might in time be able to construct a receiver that would produce the Waves in intelligible form.

It is an object; of the present invention to incre-ase the secrecy in wave transmission of the type referred to by changing from time to time the. scheme of transformation of the wave to besecretly transmitted.

lt is a further object of the invention to ,provide a large number of highly secret schemes of transformation and to facilitate shifting from one scheme to a diderent scheme of transformation.

It is a further object of the invention to facilitate and `simplify two-way transmission with secrecy.

The invention comprises a number of characteristic novel features which willappear' hereinafter in the description and claims.

Briefly and specifically described, the invention may take the form of a wire or wireless transmission system in which the waves 6o to be transmitted are first sub-dividedI by filters into sub-bands of frequency components, after which the sub-bands may be variously treated preparatory to transmission.v For example, 1n a system in which 65 the waves'are divided into four sub-bands, four modulating or wave-combining circuits are provided together with appropriate wave sources and filters for obtaining any desired one of a large number of waves possessing a high degree of secrecy, 125 such iighly secret combinations being provided by the invention. By simply throwing certain switches, a change may readily be mde from one secrecy combination to another.

This arrangement makes it practically an impossible task for an unauthorized person to decipher the message being sent, since 'the order of variation rom one secrecy combination to another may bevaried to almost an infinite degree. 4

A more detailed description of the invention will now be given 1n connection. with the accompanying drawings, in which Fig. 1 is a schematic showing of of the various elements of a wave transforming system` capable of use both in transmitting and in receivingwith secrecy; Fig. 2 shows a two-way two-station system employing the arrangement of Fig. l; Fig. 3 ao shows a two-way radio terminal employing the wave transforming arrangement of Fig. l; Fig. 4 shows details of the modulating or wave combiningci-rcuit, and Fig. 5 is a table showing in symbolic form the secrecy\ combinations according to the invention.

In Fig. 1 there is shown a wave transforming circuit leading on the left to the telephone instrument 10 and terminating on the right in the leads 50, which may be connected to any desired type of transmission system as will be explained hereinafter. This Wave transforming circuit may serve both for detecting waves originating in the telephone 10 and for incoming waves to be 105 received at 10, on .the assumption that the leads 50 are connected to a two-way system.

The speech currents originating in the telephone 10 pass into the leads 51 to which arc connected the sub-dividing or analyzing the arrangement 85 ibetransmitted and received and any desired number of sub-dividing lters may be used. It -will be obvious to proportion the transmission ranges of the respective -lters in accordance with the number of filters and thetotal frequency range covered.

'Four wave combining or modulating circuits designated M1, M2, M, and M, are associated with the respective analyzlng filters.

` These'wave combining or modulating circuits lare yfor the purpose of shifting the subbands transmitted y throu h the respective analyzing filters so that t ey will occupy a dierent range in the outgoing wave.' 'lhis will be described more fully hereinafter..

Associated with each of the modulatingcircuits M1 to M, is a group of filters Fn, Fb, i", and Fd adapted to be connected by the switches shown between the respective modulator and the outgoing or two-way circuit 50. c

Associated with each modulating circuit are a number of wave sources for supplying to the modulating circuits the waves of the proper frequencies for introducin the required frequency shift in the sub ands.

Each of the filters above referred to, as

well as those shown throughout the system,4

may be constructed in accordance with the disclosure in the patent to Campbell, No. 1,227,113, May 22, 191'?. The wave sources associated with the modulating circuits may be oscillation generators of the type shown q in the patent to Hartley, No. 1,356,763, @ctober ,26, 1920. rlhe modulating circuits may all be' constructed. alike and are preferably of the type shown more in detail in Fig. 4e.

Referring, for a moment, to Fig. li, the modulating circuit M, is shown as of the balanced tube type disclosed, for exampley in the patent to Carson, No. 1,343,306, J une 15, 1920. The oscillator 52 is connected to the common branch 53 of the input circuit of the modulator and the individual circuits are connected by means of the transformer 54 to the mid oint of the@ hybrid coil 55. yThe output si e of vthe modulator is con nected to the series winding of the coil 55.-

Assuming the frequency of oscillator y52 to be 14:00 cycles, a wave transmitted through the lter l?, is applied to the input circuit of the modulator M, along with the 1400 cycle wave and the modulator acts in' the well known manner to produce an upper side band of 1500 to 2200 cycles and a lowerl side band of 600 to 1300 cycles. These side bands are applied from the output 'of the modulator to the series winding of the repeating F, and Fd which are designed coil 55.' lt will be seen the filter incassa I that the upper side band only comprising fre uencies of 1500 to 2200 Acycles will be transmitted from the lter vFc to the outgoing circuit. lt is a Well known propert of the balanced tube modulator that it ba ances out and suppresses the unmodulated carrier component so that none of the 1400 cycles wave from the oscillator 52 is impressed on the transmission circuit. However, in case some of the unmodulated carrier component is present in the output due to sli ht unbalance, it is suppressed by C. .Waves received through the filter Fc and having frequencies of 1500 to 2200 cycles are impressed on the input circuit of the modulator M, where they com# bine with the 1400 cycle wave -to give an upper side band having frequencies extendin from 2900 to 3600 cycles and a lower si e band havin frequencies extending from 100 to 800 cyc es. rlhese side bands are v transmitted through the re. eating coil 55 and only the lower side ban is transmitted through the filter Fa to the circuit lying'to the left in the figure. The modulating circuit serves therefore, for transmission in both directions. t

Returning now to Fig. 1, it will be seen that if switch 1 is thrown to the right, switch 13 closed, and switches 102 and 106 are closed, a circuit condition exists between the uppermost filters l?, and Fc similar to, that yust described in connection with Fig.

d. 1f switch 1 is left 'thrown -to the right and other of the switches 101 to 107 are closed 1n pairs .in an a propriete manner vand alsovif the appropriate switches 11to 16 are closed, the sub-band transmitted through the lter l, may be shifted in fre'- y uency so as to be sent'outthrough any one of the hlters Fb, Fc' and Fd connected to the modulator M1. llhe frequency order of the sub-band may also be either normal or inverted. That is,"the to 800 cycle sub.- band may be stepped up as a whole in the frequencyrange in such a manner that a constant frequency is added to each of the frequency components, or, if desired, the 100 cycle component may be shifted so as to occup the i. hest frequency position of the resu tant su band and the 800 cycle component, the lowest frequency position.v l

lf the band transmitted through the filter"I l?, is, for example, to be inverted and stepped up 1n frequency so as to pass through the outgoing filter Fc the 800 cycle component of the incoming sub-band will be shifted 'upward in frequency '700 cycles and 'the incoming 100 cycle component will be shifted upward in frequency 2100 c1 les. This is accomplished by modulating aJ2300 cycle wave by the incoming sub-.band and selecting the lower side band of 'the resultant rnnodulanted wave. Similar frequency transformation may beymade in each of the other ist sub-bands by means of the modulators M2, M3 and M4 and the associated wave sources and filters.

Referring now to Fig. 5, the four subbands derived by the respective filters Fa to Fbof Fig. 1, are designated, orconvenience, by the letters, a, b, c and d; respectively. The heavy letters at the top. of the column indicate the normal frequency position of the sub-bands in the wave that is to be analyzed. rlhe letters with the accompanying arrow indicate in each horizontal i l'row of the column a diEerent combinationwhich is obtainable by merely shifting the switches that are shown in Fi 1. An arrow following-a letter and" ointing to the right indicates that the particular sub-band has a normal frequency order and a letter followed by an arrow pointing to the left means that the corresponding sub-band has an inverted frequency order. For example in the uppermost row of the .left-hand column of Fig. 5, the b Vsub-band is stepped down 1n frequency so that it occupies the normal a sub-band position and the a sub-band is stepped upward in frequency so that it occupies the normal 5 sub-band osition. Also, the c and d sub-ban s are interchanged in position, all of the frequency orders being left normal in each ofthe subbands. In the second horizontal row of the left-hand column, the same frequency shifts' are made as in the case of the first row but in addition the b sub-band is inverted in frequency.

To obtain an output wave havin the combination indicated in the first horizontal row of Fig. 5, the switches 1, 2, 3 and 4 of Fig. 1 will all be thrown tothe right. Since the a band is to be stepped up lY0U cycles in frequency, switches 11, 101 and 105 will be closed. Since the b band is to be stepped down in frequency one (position, switches 21, 201 and 205 are close Similarly, since the c hand is stepped upward one hand-width, switches 31, 303 and 307 are closed, and in order to step the d band downward, switches 41," 403 and 407 are closed.

To obtain an output wave having the combination represented in the second row of Fig. 5, the only change that is necessary from the' set-up described' for the first combination is that switch 22 instead of switch 21 is closed. The eifect of this is to modulate 1600 cycles by the b band producing a. lower side band extending from100 to 800 cycles, the 800 cycle componentbeing produced by subtracting the highest component, that is, 1500 cycles of the b band fromfthemodulating frequency so 'that the frequency order of the resultant sub-band is inverted. i

lt is to be noted that if the sub-'band Fn is to be stepped upward in frequency to occupy the next higher range with a normal frequency order, switch 11 is closed. If it is to be stepped.up to the second higher subband, switch 13 is closed, and, if itis to be stepped up to the third hi her sub-band position, switch A15 is close If the a band is to be stepped up to the next higher `sub-band and inverted the switch 12 is closed. Similarly, if it is to be stepped up two Cpositions and inverted, switch 14 is close and if it is to be stepped three steps and inverted, switch 16 is closed. i

If the b band is to be stepped downward one position, switch 21 is closed. If it is to be stepedupward one position, switch 23 is close and if it is to be stepped u ward two positions, switch 25 is i close If it is to be similarly shifted and also inverted, the corresponding switch 22, 24 or 26 is closed. If this arrangement is observed throughoutthe connections it is seen to be a very simple matter to change from .one combination to any other desired combination.

It will be noted also, that the switches for connecting the filters between the modulators and the leads are placed in horizontal and vertical rows, with four switches on each side of the filters. It will be con.- venient in practice to arrange thesev switches in some suitably manner so that, for example, by referring to a table such as that shown in Fig. 5, a corresponding switch closure will appear from the labelling. After switch 103 is closed, it will be necessary to throw switch 4c to the right. it will also be obvious that only a single pair of'switches in any horizontal level are to be closed and furthermore, that at least one pair of switches in each vertical level must be closed. This inter-relation between the various switches may readily be taken advantage of in a practical system `to facilitate the setting up of a combination which will pro duce an outgoing wave' in the leads 50 having the total frequency range of 100 to 2900 cycles and having an arrangement ingicated by one of the' horizontal rows of As stated above. the wavetransforming circuit serves 4both for vtransmitting and receiving. Forexainple, waves incoming on the line 50 will be divided intol their respective sub-bands by the lilt-ers that are connected' in4 circuit at the timebetween the leads 50 and the modulatorsv M1 td M4. These sub-bands will be impressed on the respective vmodulators in the manner described in connection with Fig. 4 and will hev combined with the appropriate waves from the sources 11 to 1G, and 21 to 20. etc., so as 'to produce and transmit through the :filters F., to Fa to the telephone 10. a'wave representing intelligible speech. The cir j iis .cuit 50, therefore, carries only waves which occupy the speech range but which do not of the message in the case of some of the` mit the chance Yunderstending o portions otthe message being transmitted,

represent normal speech.

If an unauthorized person attempts to intercept the message being transmitted. over the leads 50 to any point in the system, he will find an resented in ig. 5 unintelligible. However, by using a heterodyne method of reception and filtering arrangements to suppress por' tions of the wave,l he would, with some difficulty, be able to receive at least portions combinations indicated in Fig. 5. For eX- ample, in the case of the first combination indicated in this figure, if the observer should set a heterodyne oscillator frequency so as to receive the a sub-band correctly, he would also receive the c sub-band cor.l rectly. Of course, the c sub-band would have superposed on it the b sub-band which would be inverted, and there would be included between the 'normally received as and cy sub-bands -the d sub-bandI as noise. .Since it-is conceivable that the reception of the two sub-bands a and c in their normal position might 'ossibly pert some slight lthis combination is not counted -as che of lthe most highly secret combinations.- In a l similarmanner, in the case of thesecond combination indicated in this figure, the a and cl sub-bands could be received normal by the heterodyne method, and in the third combination, the b and d sub-bands could be similarly received. It will be-seen by inspection thateach ot the combinations numbered 1 to 125, however, is incapable ot yielding more than a single one of the subbands in its normal form for any one heterodyning frequency and these combinations are, therefore, highly secret. Aspointed out above, even inthe case of those combinations which yield two sub-bands normally, the

`presence of the other sub-bands as noise will greatly reduceV the chances ot understanding the message, so that all of thecombinations are'secret to avery considerable degree. y y

Figs. 2 and -show applications of the invention.to an ordinary telephone line and.

to a, radio system respectively. IThe rectanles WT indicate that the wave transforming system of Fig. 1 connected between the leads 50 and the telephoney 1Q is tobc inserted at this point of the circuit. The telephone line (i0 terminates at each of the two stationsshoxvvn, in a wavetranstorming circuit and in. the subscriberss'ets 61 and G2, respectively. Conversations in both' directions may be carried on between these sub scribers7 stations with secrecy'since anyone listenin@ on the line 6() will receive only unintelligilile waves.

one ot'. the combination rep-y terasse In Fig. 3, a transmitting antenna TA and a receiving antenna RA are connected through radio transmitting and receiving apparatus to the telephone line which has included between it and the subscribers set. 66,` a wave transforming circuit 1WT. The usual -two-way connection including a balancing network N for the telephone line and the conjugate transformer 67 is employed. The waves originating in the subscribers set 66 and transformed .into a secrecy combination, are transmitted through the repeating` coil 67 to the input ot' the modulator M which is supplied also by radio frequency waves from the source 68. Due to the balanced type modulator, the unmodulated carrier component is suppressed and only the two side bands of the unmodulated wave' are impressed by the antenna TA. This antenna is preferably tuned to tra-nsmit only one side band of thevmodulated wave in order to economize in the transmitted energy. The receiving antenna RA is connected to a detector D vof any suitable type supplied by la wave of the carrier frequency fromA the source 69 and the detected waves are applied from the detector to the bridged circuit of the conjugate repeating coil.. These waves' passover lthe line 65 through the wave ltransforming device where they are rendered vintelligible and are impressed on the subscribers set 66. Y

What is claimcd'is: l

l. In a secret signaling system, a circuit carrying signal waves, a plurality of yfilters connected to said circuit for selectively transmitting respectively dierent trequency components of the signal, wave com- ,biningmircuits associated with said filters, a

rality of sources of waves of different fre# I quencies, wave combining circuits adaptedy to be associated with said filters and with said sources for combining said waves and said bands, other filtersbetween said wave combining circuits and said outgoing cir- '125 cuit for separating the combined waves iut-o bamds of frequency components, and switchinv" .means for selectingiand associating with sald voutgglgolng circuit various combinations of the first mentioned lfilters, said wave .130

combining circuits and said other filters, to produce different Vdesired combinations ofr frequency-bands to make up the waves transmitted into the outgoing circuit.

3. In a secret signaling system, lan incoming circuit, an 'outgoingk.circuit, filters for selecting bands of frequency components from waves'in'the incoming c'ircuit, means to shift the frequencies of the bands to cause each shifted band to occupy the frequency range occupied by a different band of coinponents of the incoming Wave, switching means for changing the scheme of shiftingof the several bands, and means to impress the shifted bands on the outgoing circuit.

4. In a secret signaling system, an incoming circuit, an outgoing circuit, filters for selecting bands of frequency components1 from waves in the incoming circuit,means to shift the frequencies of the bands to cause eachshifted band to occupy the frequency range occupied by a different band of components of the incoming wave and to invert. the frequency order of the components within certain of the shifted bands, switching means for changing the scheme of shifting and inverting of the several bands, and means to impress the shifted bands on the outgoing circuit.

5.. In a signalingsystem, a two-way transmission circuit, means to impress waves having a band of frequency-components upon said circuit, means interposed in said circuit for sub-dividing the waves being transmitted in either direction over said circuit into a plurality of sub-bands of frequency components, means for interchanging the ositions of the sub-bands in respect to the requency ranges which the occupy, and means to impress the shifte sub-bands on said circuit.

6. In a signaling system, a circuit for carrying signaling waves having a band of frequencies, four filters in said circuit for analyzing the waves into four different subbands of frequency components, a plurality of sources of waves of different frequencies, wave combining circuits, and ymeans comprising a pluralityof switches for selecting and associating -said filters, said sources and said wa ve combining circuits in various combinations so as to interchange the frequency position of said sub-bands and to invert the frequency order ofthe :components within the individual sub-bands in the combined waves, thereby producing any desired one of two hundred and eight compositions in the resultant waves.

7. In combination, a modulator, means to I impressl a band of frequency, components upon said modulator, a band filter connected to the output side o f said modulator, a pair Iof wave sources of respectively different frequency associated with said modulator, and means to connect either wave source to said modulator at will, the frequency of onc wave source-being such that the upper side band of the modulated wave resulting from modulating the wave by said band of components passes through said filter to the exclusion of the lower side band, and the frequency of the other wave source being such that the lower side band resulting from modulating the wave by said band of components passes through the vfilter to the exclusion of the upper side band, the first mentioned upper side band and the last mentioned lower side- -within the limits of the speech band in accordance with any one of a plurality of schemes to unintelligib e wave occupying substantially the normall speech frequency range, and switching means to shift from one scheme of alteration to another at will.

In witness whereof, I lhereunto subscribe my name this 11 day of July A. D., 1923.

HARVEY FLETCHER.

reduce in each case a resultant 

